frontend: Highlight first, not last error in line

We highlight the offending token with an error message, but due to the
limited data structure (don't feel like refactoring now, plus
performance is already an issue) - we only allow for one token
highlight per error lines. Typically the first error is the most
meaningful, whereas later errors are sometimes just follow-ons - so
only highlight the first error.

Author: juliaogris

frontend/index.js

@@ -120,7 +120,9 @@ function jsError(ptr, len) {
   const errorLines = {}
   for (const err of errs) {
     const g = err.match(re).groups
-    errorLines[g.line] = { col: g.col, text: lines[g.line - 1] }
+    if (!errorLines[g.line]) {
+      errorLines[g.line] = { col: g.col, text: lines[g.line - 1] }
+    }
     msgs += `line ${g.line}: ${g.msg}\n`
   }
   const output = document.querySelector("#console")